brake booster
Thread Starter
Le Mans Master
Joined: Oct 2006
Posts: 8,940
Likes: 35
From: El Cajon California
brake booster
My brakes goes to the floor slowly, I replaced the master cylinder about eight months ago so I took it back and they swapped me out with another.
I bled the brakes and they felt firm but when I started the car and drove around the block the brakes went to the floor again, do I have bad brake booster?
I bled the brakes and they felt firm but when I started the car and drove around the block the brakes went to the floor again, do I have bad brake booster?
Melting Slicks
Joined: Jun 2006
Posts: 2,071
Likes: 147
From: S Nevada
No. Even with no vacuum on the car you should be able to get a firm pedal. At no point should it go to the floor. You either have another bad master cylinder, a leak in a line or caliper, or you haven't removed all the air from the system. You know that the rear calipers have 2 bleed screws each right? Bleed inbd bleeders first then outbd. If bleeding again still doesn't help, how old are your rubber brake hoses? I wouldn't think they could allow the pedal to go completely to the floor, but can cause a spongy pedal.
Last edited by Sunstroked; Jul 5, 2014 at 01:45 PM.
Thread Starter
Le Mans Master
Joined: Oct 2006
Posts: 8,940
Likes: 35
From: El Cajon California
I disconnected the vacuum line to the booster and capped it, the pedal is nice and firm, I can drive is around the block just fine except I have to press the pedal very hard!
Mrvette I will searchback farther in history
Mrvette I will searchback farther in history
Melting Slicks
Joined: Jun 2006
Posts: 2,071
Likes: 147
From: S Nevada
Snoopysvet, that is what I was saying, even with no vacuum you should still be able to get a firm pedal. But.... If your pedal still goes to the floor while depressing and holding it, you still have a problem. These 2 photos show a brake booster from my corvette. The shafts protruding from each end mate up inside of the booster, separated by the diaphragm. Essentially forming a solid link. Each shaft on opposing sides. As you depress your pedal with a properly working diaphragm, the vacuum assists in the actuation, yet the two shafts are still mated up. If it were any other way, failure of the booster would result in loss of brakes. Not good for anyone.
Last edited by Sunstroked; Jul 6, 2014 at 12:37 PM.
Safety Car
Joined: Sep 2005
Posts: 4,221
Likes: 643
From: Orange County Ca
So, with the booster hose disconnected, the pedal stays high. When the booster hose is connected, the pedal goes down. This tells me that the booster is fine. The extra pressure allows the real problem to show up. Master cylinder is my best guess, then air in the lines, hoses, etc as stated above.
I personally have had newly rebuilt mc's right out of the box be bad. I have a new AC Delco (China made) that is going bad after less than 1k miles.
I've made the mistake in the past in thinking that once I replaced a part, that part was good, the therefore, something else was wrong. This is no longer true.
I personally have had newly rebuilt mc's right out of the box be bad. I have a new AC Delco (China made) that is going bad after less than 1k miles.
I've made the mistake in the past in thinking that once I replaced a part, that part was good, the therefore, something else was wrong. This is no longer true.
Racer
Joined: May 2016
Posts: 359
Likes: 153
From: Clinton, Illinois
Hi there,
I had much the same problem. My pedal after driving a few miles would go to the floor. I'll bet I bled two gallons of fluid through the system over a 2 week period, and there were absolutely no leaks, but when bleeding there was always air in the driver side rear caliper. So where was the air coming from? I checked everything the great folks on the forum said to check...and found nothing. I changed the booster, master cylinder then repeated the bleeding process and the brakes would still somehow get air into the system. This was a real head scratcher. I talked to a company called V-Tech Corvette in Rockford Il. He suggested to check the rear wheel bearings. Well what do you know, the left rear wheel bearing was totally fried and the runout of the rotor was far beyond factory spec. The stock calipers have a piston seal that is called a "lip" seal. The excessive runout caused the seal to fail, not by leaking but "sucking" air into the lines through the seal. You need to check the runout and wheel bearings. My guess is that your problem lies somewhere in there. Hope you get the problem solved...good luck!
I had much the same problem. My pedal after driving a few miles would go to the floor. I'll bet I bled two gallons of fluid through the system over a 2 week period, and there were absolutely no leaks, but when bleeding there was always air in the driver side rear caliper. So where was the air coming from? I checked everything the great folks on the forum said to check...and found nothing. I changed the booster, master cylinder then repeated the bleeding process and the brakes would still somehow get air into the system. This was a real head scratcher. I talked to a company called V-Tech Corvette in Rockford Il. He suggested to check the rear wheel bearings. Well what do you know, the left rear wheel bearing was totally fried and the runout of the rotor was far beyond factory spec. The stock calipers have a piston seal that is called a "lip" seal. The excessive runout caused the seal to fail, not by leaking but "sucking" air into the lines through the seal. You need to check the runout and wheel bearings. My guess is that your problem lies somewhere in there. Hope you get the problem solved...good luck!
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Hi there,
I had much the same problem. My pedal after driving a few miles would go to the floor. I'll bet I bled two gallons of fluid through the system over a 2 week period, and there were absolutely no leaks, but when bleeding there was always air in the driver side rear caliper. So where was the air coming from? I checked everything the great folks on the forum said to check...and found nothing. I changed the booster, master cylinder then repeated the bleeding process and the brakes would still somehow get air into the system. This was a real head scratcher. I talked to a company called V-Tech Corvette in Rockford Il. He suggested to check the rear wheel bearings. Well what do you know, the left rear wheel bearing was totally fried and the runout of the rotor was far beyond factory spec. The stock calipers have a piston seal that is called a "lip" seal. The excessive runout caused the seal to fail, not by leaking but "sucking" air into the lines through the seal. You need to check the runout and wheel bearings. My guess is that your problem lies somewhere in there. Hope you get the problem solved...good luck!
I had much the same problem. My pedal after driving a few miles would go to the floor. I'll bet I bled two gallons of fluid through the system over a 2 week period, and there were absolutely no leaks, but when bleeding there was always air in the driver side rear caliper. So where was the air coming from? I checked everything the great folks on the forum said to check...and found nothing. I changed the booster, master cylinder then repeated the bleeding process and the brakes would still somehow get air into the system. This was a real head scratcher. I talked to a company called V-Tech Corvette in Rockford Il. He suggested to check the rear wheel bearings. Well what do you know, the left rear wheel bearing was totally fried and the runout of the rotor was far beyond factory spec. The stock calipers have a piston seal that is called a "lip" seal. The excessive runout caused the seal to fail, not by leaking but "sucking" air into the lines through the seal. You need to check the runout and wheel bearings. My guess is that your problem lies somewhere in there. Hope you get the problem solved...good luck!